The invention relates to forming optical gratings. More specifically, the invention relates to methods for forming optical gratings which are readily adaptable to use in connection with weak fiber Bragg gratings and to automated fabrication techniques.
Fiber Bragg gratings are optical fibers that have been modified by modulating the longitudinal index of refraction of the fiber core, cladding or both to form a pattern. A fiber Bragg grating functions to modify the optical passband of the fiber (transmission characteristic) in such a way as to only transmit a selected and controlled wavelength band and to reflect other wavelengths.
Fiber Bragg gratings have been applied to optical communications systems for Wavelength Division Multiplexing (WDM) and in various other applications, such as reflectors and filters. One known application for fiber Bragg gratings is the tuning, i.e. stabilization, of lasers. For example, it is well known to place a weak fiber Bragg grating within the fiber of a 980 nm amplifier pump or a 1480 nm amplifier pump to stabilize the output wavelength.
Because of the plurality of variables which determine the transmissive and reflective characteristics of a fiber Bragg grating, such as the dimensions and refractive index of affected portions of the fiber, it is difficult to determine the characteristics of the grating prior to manufacturing the fiber Bragg grating. Therefore, the characteristics of fiber Bragg gratings ordinarily are determined by measuring the transmission characteristic of the fiber Bragg grating while forming, i.e., writing, the grating in the fiber and terminating the forming process when desired characteristics are obtained. Various processes for forming the gratings are well known. For example, the xe2x80x9cholographic methodxe2x80x9d of forming a fiber Bragg grating comprises irradiating the fiber core from one side with a UV two beam interference pattern thus forming a permanent periodic refractive index modulation along the fiber axis by changing the refraction index of irradiated portions of the fiber.
However, laser stabilization techniques typically require reflection coefficients of the fiber Bragg grating on the order of 2%-4%, i.e. a xe2x80x9cweak fiber Bragg gratingxe2x80x9d. Such low, i.e. xe2x80x9cweakxe2x80x9d reflection coefficients result in transmission levels of only xe2x88x920.08 dB to xe2x88x920.18 dB. Spectral ripple on a typical commercially available 980 nm source of the type typically used to monitor fiber Bragg gratings is on the order of 0.2 dB or less per nm span. Accordingly, extracting a grating transmission signal from the source spectrum is difficult because it requires a very accurate reference spectrum. Modal interference further compounds this problem.
Nothwithstanding the above, the characteristics of fiber Bragg gratings can often be measured during grating fabrication using the above-noted transmissive method with a transmission reference taken immediately prior to formation of the grating. However, fiber Bragg gratings require an annealing cycle to complete formation. The annealing cycle and the usual subsequent need for a new optical connection to the grating ordinarily affect the absolute transmission reference for the grating. Therefore, the characterization of the grating after annealing often is not accurate.
A first aspect of the invention is a method for manufacturing an optical grating on a length of optical fiber having input and output ends, comprising the steps of conducting guided light into the input end of the fiber and measuring light reflected back from the output end with an optical sensor, forming an optical grating in the fiber between the input and output ends, conducting guided light into the input end of the fiber and measuring light reflected back from the grating with an optical sensor, and comparing measured light to determine a percentage of guided light reflected back from the grating.
A second aspect of the invention is a method of manufacturing grating on a length of optical fiber having input and output ends, comprising the steps of, conducting guided light into the input end of the fiber and measuring light reflected back from the output end, defining an optical terminator on the fiber, writing a grating in the fiber between the input end and the optical terminator, conducting guided light into the input end of the fiber and measuring light reflected back from the grating, and comparing the measured light to determine a characteristic of the grating.